Hey, fellow Tennessean,
If you book through Carnival, the times will not be a problem. My husband & I did the Chichen Itza tour in May. We had a great time. You get off the ship about 7 AM, walk down the pier & through a store. When you reach the other side, there will be signs directing you where to go. There will be a lot of people getting off the ship at that time & possibly another ship as well, but just go with the flow & everything will be fine. There will be several buses going to Chichen Itza. We got back to the ship with time to spare. However, if there is a problem & the bus gets back late, as long as you booked through Carnival, the ship will wait for you. If you know you want to do Chichen Itza, I would go ahead & book online(when you can), otherwise you will have to stand in line at the shore excursions desk once onboard. If you aren't sure which excursion you want, you can wait until you get on board. There is a small chance that the tour could become full if you wait, but I doubt it. The Cruise Director does a shore excursion talk during the first day at sea that explains what each excursion is. For more info on the Chichen Itza tour, read here:
http://www.carnivalconnections.com/cruisetalk/forums/p/35999/98739.aspx#98739
Happy Cruising!